Recently, use of a semiconductor laser or a YAG laser is increasing as an image recording laser in view of stability, compact and ease of maintenance. The technique shifting the wavelength of these lasers to shorter wavelength has been sought, and a laser (532 nm) employing SHG of a YAG laser is put into practical use. A method of forming an image is known which comprises the steps of exposing a photopolymerizable composition to a laser such as a semiconductor laser or a YAG laser to cause radical polymerization to harden at exposed portions, whereby difference of physical property between the exposed and unexposed portions is produced, and then to develop to form an image.
In order to form an image employing a laser exposure, a light sensitive composition suitable for a laser light wavelength is necessary, and a radical generating agent as a radical polymerization initiator has been studied. Most radical generating agents alone have absorption only at ultra violet wavelength regions. The agents are ordinarily used in combination with a sensitizing dye capable of absorbing a light emitted from a light source.
Various combinations (hereinafter referred to also as initiator compositions) of a sensitizing dye for a 488 nm argon laser or a 532 nm W-YAG laser with a radical generating agent are known. A specific photo-reducing dye as an effective sensitizing dye is described in U.S. Pat. No. 2,850,445. An initiator composition comprising a dye and amines is disclosed in Japanese Patent Publication No. 44-20189, an initiator composition comprising a biimidazole and a radical generating agent in Japanese Patent Publication No. 45-37377, an initiator composition comprising a biimidazole and dialkylaminobenzylideneketones in Japanese Patent Publication No. 47-2528 and Japanese Patent O.P.I. Publication No. 54-55292, an initiator composition comprising a keto-substituted cumarin and an active halogenated compound in Japanese Patent O.P.I. Publication No. 58-15503, and an initiator composition comprising a substituted triazine and a merocyanine dye in Japanese Patent O.P.I. Publication No. 54-15102. However, the above techniques have problems in sensitivity and storage stability.
A dye having a pyrazolo 1,5-b!1,2,4! triazole nucleus for a photographic material is disclosed in Japanese Patent O.P.I. Publication No. 60-213937, but there is no description therein that a combination of the dye with a radical generating agent can be used as a photopolymerization initiator. Further, there is no description therein of a dye having a pyrazolo 3,2-b!1,2,4! triazole nucleus. It is unexpected that these dyes have a capability of generating a radical.